Transmission lines are difficult to drive with integrated circuit (IC) buffers. The relatively small size of the IC transistors produce a limited amount of current to drive the relatively long transmission line. As IC devices shrink, the length of transmission lines tend to remain constant, increasing the relative mis-match. Also, higher speed transitions are more prone to undesirable effects such as distortion and ringing.
FIG. 1A shows a prior art transmission line. The transmission line is 40 inches long in this example. Transmitter 102 drives transmission voltages VTx onto the near end of the transmission line. Signals caused by transitions of VTx travel down the transmission line and reach receiver 104 as received voltages VRx. The relatively long length of the transmission line causes distortions in the received signals, such the attenuation of high frequency pulses being larger than the attenuation of low frequency pulses, and the signal delay differs for different frequencies. Thus both amplitude and phase distortion occur at the received voltage VRx.
In FIG. 1B, a redriver is placed on the transmission line. Redriver 106 is placed midway on the transmission line, about 10 inches from transmitter 102 and 30 inches from receiver 104 in this example. Redriver 106 can cancel phase distortion due to the transmission line between 102 and 106. Hence redriver 106 can extend the total allowable distance between the 102 and 104.
Redriver 106 normally has a non-linear limiting amplifier that has a fixed or limited output swing. The pulse with different frequencies that is transmitted by redriver 102 may not the have same swing because of the pre-emphasis or de-emphasis setting. However, the signal VRx received by redriver 106 from transmitter 102 is amplified by non-linear redriver 106, and the maximum voltage of all frequencies is often reached, causing voltages to be limited or clipped on the output of redriver 106, signal VTx2. This means that the output amplitude of the pulse with different frequencies at the output of redriver 106 is the same or similar and the swing at the output of redriver 106 is independent of the input swing. Hence, the input amplitude information is lost.
Transmitter 102 may send a training sequence of pulses or other signals on VTx to receiver 104. The amplitude of the transmitted signal from transmitter 102 may be varied during training to determine the optimal amplitude to use. Other transmission characteristics may also be varied during training, such as an amount of pre-emphasis or de-emphasis, signal frequency, etc.
During the training sequence, the distortion caused by the transmission line is detected by receiver 104, and transmitter 102 may adjust its transmission characteristics, such as output swing, pre-emphasis/de-emphasis, signal frequency and signal pattern. However, when redriver 106 is inserted between transmitter 102 and receiver 104, these training pulses may be clipped by the voltage-limiting amplifier in redriver 106. Thus the amplitude sent by transmitter 102 may be limited or clipped by redriver 106, causing receiver 104 to receive training pulses at VRRx with clipped peaks or other distortions introduced by redriver 106.
FIG. 2 is a waveform diagram of simplified training pulse on a transmission line. Transmitter 102 sends training pulses as variations in voltage VTx. Typically, the insertion loss of the transmission line is proportional to the frequency, so the amplitude at higher frequencies is normally less than the amplitude at lower frequencies. To compensate for this effect, transmitter 102 increases the drive current at higher frequencies, causing the near-end voltage VHF for high frequency to be larger than the near-end voltage for low frequency VLF. Thus transmitter 102 boosts the output amplitude at high frequency (VHF) relative to that of low frequency (VLF). For example, transmitter 102 may increase the amplitude by 50% at high frequency, so that the ratio of VHF/VLF=1.5, as measured at the output of transmitter 102, transmitted voltage VTx.
The received voltage VRx at the far end of the transmission line that is input to receiver 104 has a reduced amplitude due to losses on the transmission line. Thus VHF is reduced by a factor A at receiver 104. Also, the ratio of VHF/VLF is reduced to about 0.6 because the optimum trace length for transmitter 102 is 30 inches, for this example. Thus while ratio of high and low frequency amplitude is 1.5 at the output of transmitter 102, it falls to a ratio of 0.6 at the input to receiver 104.
In addition to a reduction in amplitude along the length of the transmission line at high frequencies, phase also distorted. The phase delay of the received signal VRx is dependent on the signal frequency, which is due to capacitive and other loading effects of the transmission line. For example, the phase delay at high frequency P1 may be smaller than the phase delay at lower frequency P2, when measured at the input to redriver 106, signal VRx.
When redriver 106 is inserted between transmitter 102 and receiver 104, the input to redriver 106 is voltage VRx. Phase distortions are reduced by redriver 106, so that P1 is about equal to P2 on the output from redriver 106, signal VTx2, but amplitude distortions remain. Hence, the ratio of VHF/VLF is about 0.8 (less than 1.0) at the VRRx input of receiver 104 even though transmitter 102 can drive VTx to an optimum of 30 inches of trace length, and redriver 106 can restore the phase distortion due to 10 inches of transmission line.
What is desired is a trace canceller that does not limit amplitude. A trace canceller that is inserted on a transmission line between a transmitter and a receiver is desired to cancel trace distortions of both phase and amplitude.